Three-Component Suzuki–Knoevenagel Synthesis of Merocyanine Libraries and Correlation Analyses of Their Oxidation Potentials and Optical Band Gaps

The Suzuki coupling Knoevenagel condensation one-pot synthesis of boronic acids/esters, (hetero)aromatic bromo aldehydes and methylene active compounds is a highly practical consecutive three-component process to provide substance libraries with 60 donor-π-bridge-acceptor molecules, i.e., merocyanines in a broader sense, in moderate to excellent yield. As already seen with the naked eye, a broad variation of the optical properties becomes accessible using this practical synthetic tool. More systematically, correlation analyses upon plotting the optical band gaps against the first oxidation potentials of redox active systems of consanguineous series furnishes linear correlations and, by extension, two parameter plots (oxidation potential and emission maximum) planar correlations with the optical band gaps.


Introduction
Multicomponent reactions (MCR) [1][2][3][4], conducted in domino, sequential or consecutive fashion, represent a powerful, efficient and efficacious tool for constructing complex molecular scaffolds in a one-pot fashion. In recent years, we have developed and explored MCRs as concise entries to functional chromophores. In particular, for fluorophores and electrophores [5][6][7], MCRs are most fruitfully applied in the sense of a chromogenic approach, which results directly in the formation of the functional chromophore of interest.
Since merocyanines are outstandingly important in OPV [19], the effect of acceptor strength in molecular architectures on the morphology of bulk-heterojunction fullerene-free solar cells [32] as well as their theory-based definition of donor and acceptor strength in conjugated copolymers [33] are of general relevance. Therefore, we assumed that a diversity oriented one-pot methodology could not only provide libraries of various consanguineous diversity oriented one-pot methodology could not only provide libraries of various consanguineous series of merocyanines, but also could provide access to systematic correlation analyses. Here, we present the methodological extension of our recently reported consecutive Suzuki-Knoevenagel three-component synthesis to a broad range of merocyanines, their electronic data by photophysics (absorption and emission spectroscopy) and cyclic voltammetry, as well as several established structure-property relationships by correlation analyses between the compounds' oxidation potentials and their optical band gaps.

Synthesis
Recently, we have established the one-pot Suzuki-Knoevenagel condensation (SuKnoCon) sequence for synthesizing carboxylic acid functionalized merocyanines with tunable donors for DSSC studies [31]. We reasoned that this general, straightforward principle of concatenating Suzuki arylation and Knoevenagel condensation in a consecutive three-component fashion can be particularly favorable for accessing substance libraries of dyes for establishing structure-property relationships based upon correlation analysis (Scheme 1). Scheme 1. Multicomponent synthetic concept based upon a Suzuki-Knoevenagel sequence.

Electronic Characteristics
The electronic properties of the merocyanines 8-12 were determined by recording the absorption and emission spectra as well as the cyclic voltammograms in the anodic regions (oxidations) ( Table 1). While oxidation potentials reflect a property of the electronic ground state of the chromophores, their absorption and emission behavior correlate to the photonic transitions from the electronic ground to the excited states and back. The oxidation potential of the compounds predominantly depends on the presence of a readily oxidizable moiety in the π-bridge, such as 4-octyl thienyl, carbazole or phenothiazine units, or in the substituent R 1 . As in our previous study [31], we calculated the half-wave potentials E1/2 referenced against the normal hydrogen electrode by adding 0.2 V to the measured value of E0 (against Ag/AgCl). The first oxidations were mostly obtained as electrochemically reversible waves in a range from 0.79 to 1.71 V. For irreversible potentials, only the anodic peak potential was documented. The obtained merocyanines display longest wavelength absorption maxima λmax,abs covering a broad part of the UV/V is spectrum ranging from 367 to 580 nm. As seen from the molar decadic extinction coefficients, these transitions are quite intense and account to charge transfer character from the donor to the acceptor moiety, as previously corroborated for phenothiazine-based DSSC merocyanines [31]. In addition, most of the compounds reveal emission maxima λmax,em ranging from 412 to 668 nm, which were not quantified due to very variable intensity. Nevertheless, Stokes shifts Δ, i.e., energy differences between longest wavelength absorption maxima and emission maxima, as an indicator of changes in the electronic structure upon photonic excitation from the ground state to the vibrationally relaxed excited states were calculated in a range between 1200 and 8000 cm −1 (0.147-0.990 eV). In addition, from the absorption and emission maxima the optical band gap, i.e., the E0-0 transition, was estimated by the arithmetic average of the corresponding and ranging from 2.083 to 3.197 eV. For electronic comparison and for establishing acceptor parameters (vide infra) phenothiazine aldehyde 13 was reacted by Knoevenagel condensation with methylene active compounds 7a, 7c, 7e, and 7f to give merocyanines 14-17 in excellent yield (Scheme 4).

Electronic Characteristics
The electronic properties of the merocyanines 8-12 were determined by recording the absorption and emission spectra as well as the cyclic voltammograms in the anodic regions (oxidations) ( Table 1). While oxidation potentials reflect a property of the electronic ground state of the chromophores, their absorption and emission behavior correlate to the photonic transitions from the electronic ground to the excited states and back. The oxidation potential of the compounds predominantly depends on the presence of a readily oxidizable moiety in the π-bridge, such as 4-octyl thienyl, carbazole or phenothiazine units, or in the substituent R 1 . As in our previous study [31], we calculated the half-wave potentials E 1/2 referenced against the normal hydrogen electrode by adding 0.2 V to the measured value of E 0 (against Ag/AgCl). The first oxidations were mostly obtained as electrochemically reversible waves in a range from 0.79 to 1.71 V. For irreversible potentials, only the anodic peak potential was documented. The obtained merocyanines display longest wavelength absorption maxima λ max,abs covering a broad part of the UV/V is spectrum ranging from 367 to 580 nm. As seen from the molar decadic extinction coefficients, these transitions are quite intense and account to charge transfer character from the donor to the acceptor moiety, as previously corroborated for phenothiazine-based DSSC merocyanines [31]. In addition, most of the compounds reveal emission maxima λ max,em ranging from 412 to 668 nm, which were not quantified due to very variable intensity. Nevertheless, Stokes shifts ∆ṽ, i.e., energy differences between longest wavelength absorption maxima and emission maxima, as an indicator of changes in the electronic structure upon photonic excitation from the ground state to the vibrationally relaxed excited states were calculated in a range between 1200 and 8000 cm −1 (0.147-0.990 eV). In addition, from the absorption and emission maxima the optical band gap, i.e., the E 0-0 transition, was estimated by the arithmetic average of the corresponding and ranging from 2.083 to 3.197 eV.
A qualitative look at the electronic data (E 1/2 , λ max,abs , λ max,em , Stokes shift ∆ṽ, and E 0-0 ) shows that the oxidation potential E 1/2 reflecting the donor strength is affected by the acceptor strength and a qualitative alignment acceptors with decreasing strength concomitantly correlates with an increasing optical band gap E 0-0 ( Figure 2). In the same trend absorption and emission bands are blue shifted (to shorter wavelength) with decreasing acceptor strength. For the unperturbed reference merocyanines 14-17, the plots of λ max,abs , λ max,em and E 0-0 (as energies in eV) against E 1/2 (in Volt) give reasonably good linear correlations (r 2 > 0.90) (for details on correlation analyses, see Supplementary Material). Since the series 9, 11 and 12, maintaining the constant donor moiety while varying the acceptor parts, give poor correlations, we instead considered variations of the donor parts while maintaining the corresponding acceptors constant as grouped in Table 2.          A qualitative look at the electronic data (E1/2, λmax,abs, λmax,em, Stokes shift Δ, an shows that the oxidation potential E1/2 reflecting the donor strength is affected by the tor strength and a qualitative alignment acceptors with decreasing strength concom correlates with an increasing optical band gap E0-0 ( Figure 2). In the same trend abso and emission bands are blue shifted (to shorter wavelength) with decreasing ac strength. For the unperturbed reference merocyanines 14-17, the plots of λmax,abs, λmax E0-0 (as energies in eV) against E1/2 (in Volt) give reasonably good linear correlation 0.90) (for details on correlation analyses, see Supplementary Material). Since the serie and 12, maintaining the constant donor moiety while varying the acceptor parts, giv correlations, we instead considered variations of the donor parts while maintaining t responding acceptors constant as grouped in Table 2. The consanguineous acceptor series of 3-methyl-4-oxo-2-thioxothiazolidin-5-y 1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene, 3-methyl-5-oxo-1-phenyl-1,5-dihydro-4 razol-4-ylidene and cyano(4-nitrophenyl)-methylene derivatives gave good to ex linear correlations for the plots of λmax,abs, λmax,em, Stokes shift Δ̃ and E0-0 (as energies against E1/2 (in Volt) (r 2 > 0.90) (for details on correlation analyses, see Supplementa terial). In particular, the excellent linear correlations (r 2 = 0.943-0.999) between the tion potential E1/2 and the optical band gap E0-0 ( Table 3)) suggest that the ele ground state property, i.e., oxidation potential, affects the photonic property, i.e. gap, and could lead to a more general correlation beyond consanguineous chromo series. Therefore, we expanded the basis of chromophores to a total of 24 (for corr analyses, see Supplementary Information). An attempt to establish a direct relati tween E1/2 and E00 gave only a poor linear correlation (r 2 = 0.7338).

Materials and Methods
All synthetic details on the preparation as well as the 1 H and 13 C NMR spectra of the series 8, 9, 10, 11 and 12, and the reference chromophores 14-17 are compiled in the Supporting Information.

Materials and Methods
All synthetic details on the preparation as well as the 1 H and 13 C NMR spectra of the series 8, 9, 10, 11 and 12, and the reference chromophores 14-17 are compiled in the Supporting Information.

Materials and Methods
All synthetic details on the preparation as well as the 1 H and 13 C NMR spectra of the series 8, 9, 10, 11 and 12, and the reference chromophores 14-17 are compiled in the Supporting Information.

Materials and Methods
All synthetic details on the preparation as well as the 1 H and 13 C NMR spectra of the series 8, 9, 10, 11 and 12, and the reference chromophores 14-17 are compiled in the Supporting Information.

Synthesis of Compound 9d by Coupling-Condensation One-Pot Synthesis (Typical Procedure)
4-Methylphenylboronic acid (6b) (150 mg, 1.10 mmol), 5-bromothiophene-2-carbaldehyde (2) (190 mg, 1.00 mmol), cesium fluoride (486 mg, 3.20 mmol) and tetrakis(triphenylphosphane)palladium(0) (24 mg, 0.02 mmol) were placed in a Schlenk flask with magnetic stir bar under nitrogen and dry 1,4-dioxane (4 mL) were added. The solution was heated to 100 °C under reflux for 8 h. After cooling to room temp acetic acid (2 mL), However, plotting E 0-0 vs. the two parameters E 1/2 and λ max,em representing both ground and excited state energetics give a quite good planar correlation (r 2 = 0.93504), were the slopes indicate that the emission λ max,em contributes to a larger extent than the oxidation potentials ( Figure 3). While the oxidation potential represents an electronic ground state parameter the emission, mostly resulting from radiative deactivation of the vibrationally relaxed excited state S 1 depends on the electronic structure of the excited state. For merocyanines typical are highly polar excited states, which attribute for a distinct degree of charge transfer character. This planar correlation now allows for a series of merocyanines to predict optical band gaps from first oxidation potentials and emission maxima.

Materials and Methods
All synthetic details on the preparation as well as the 1 H and 13 C NMR spectra of the series 8, 9, 10, 11 and 12, and the reference chromophores 14-17 are compiled in the Supporting Information.